Three people were sent to the hospital after a portion of an Interstate 5 highway bridge in Mount Vernon, Wash., collapsed Thursday, dumping two vehicles and a travel trailer into the icy water, authorities said.

The three people were rescued from the Skagit River by first responders and taken to local hospitals with non-life threatening injuries.

Officials located a semi-truck believed to have hit several girders on the four-lane bridge just before the collapse. The driver remained on the scene and was cooperating with investigators, Washington State Patrol Chief John Batiste said at a press conference early this morning.

"We're looking at the cause being an oversized, over-height vehicle, striking critical portions of this bridge, causing it to collapse," said Travis Phelps of the Washington State Department of Transportation and Washington State Patrol.

The National Transportation Safety Board will arrive later today to investigate the collapse.

The collapse occurred on the portion of Interstate 5 over the Skagit River, about 60 miles north of Seattle.

Dan Sligh and his wife, Sally, were among the injured transported to Skagit Valley Hospital in Mount Vernon. Sligh told ABC News affiliate KOMO-TV that he was treated for cuts, bruises and a separated shoulder. Sligh's wife has internal bleeding and will need to stay overnight.

Sligh was driving his truck with a travel trailer attached to it to begin a camping trip. Sligh said he was crossing the bridge behind the semi-truck when the accident occurred around 7 p.m. local time.

"I was commenting to my wife that it seemed that the load he was carrying was about 4 feet wider than the actual bridge," he said.

The vehicles plunged about 40 feet from the bridge into the river, which set off a massive rescue operation.

"It was just a white flash and cold water," Sligh said. "The Skagit is quite cold this time of year."

Sligh said he acted quickly to rescue his wife, who was unresponsive after the collapse.

"Popped my shoulder back in so I could unbuckle everything so I could get over to her. Unbuckled her and pulled her into my side which had less water," he said.

Helicopter footage from ABC News affiliate KOMO-TV showed several rescue boats in the Skagit River with several ambulances waiting on the shore.

"When you're sitting down in the water and there's all that mangled metal and bridge and you're looking around kind of pinching yourself and realizing you're lucky to be alive ... it's a pretty amazing day to tell you the truth," Sligh said.

The bridge, built in 1955, was not considered structurally deficient but was listed as "functionally obsolete" - a category indicating an outdated design, such as having narrow shoulders and low clearance underneath, according to a database compiled by the Federal Highway Administration.

Federal records show it had a sufficiency rating of 57 out of 100, meaning it's in need of repairs. The bridge was inspected twice last year, most recently in November, and repairs were made, according to Washington Transportation Secretary Lynn Peterson

Clean-up efforts will take several days to weeks, according to Phelps. The bridge sees 77,000 cars per day, and Phelps said they were expecting significant congestion until the bridge is fixed.

Washington Gov. Jay Inslee told reporters that one in five bridges in Washington have a rating of "functional obsolescence," which he described as "troubling." Inslee acknowledged the bridge collapse is going to cause a headache for tens of thousands of drivers.

"This is the aorta, the arterial of commerce for western Washington and we will ask all Washingtonians to help us avoid traffic problems," he said.

I-5 is the longest interstate highway on the West Coast, running from the Mexican border all the way north to Canada.